The invention relates to a brushless three-phase current generator having a fed-back, controllable device for self-excitation and automatic control of the energizing current, said device comprising controllable means for deriving from the generator output part of the output voltage and the load current of the generator and the vectorial combination and supply thereof to a multi-phase rectifier whose output is connected to a stationary d.c. energizing winding of an excitor, the rotating multi-phase winding of which enables the rotating field winding of the generator through a commonly rotating rectifier. Such a generator is known from the U.S. Pat. No. 3,605,006.
The brushless current supply to the rotating field winding of the generator from the multi-phase winding of an excitor having a stationary d.c. energizing winding through a commonly rotating rectifier involves various disadvantages. Assuming the excitor voltage to be proportional to the excitor field current and the number of revolutions and the generator voltage to be proportional to the generator field current and the number of revolutions, the number of revolutions has a square effect on the relationship between the excitor field current and the generator voltage. If the number of revolutions is slightly too low, the excitor field winding is exposed to the risk of burning. A further problem resides in the adverse effect of the threshold voltage of the two rectifiers on the voltage building up, which will practically always require an auxiliary device. Moreover, the magnetic time constants of the excitor field and the generator field have a delaying effect on the adjustment of the voltage in the event of rapid load variations.
The invention has for its object to obviate the aforesaid disadvantages of the brushless embodiment. According to the invention this is achieved by a synchronous reactance of the multi-phase winding of the excitor at the frequency corresponding to the nominal speed, which is greater than the equivalent resistance of the generator field winding. Owing to the predominant influence of the reactance the generator field current is proportional to the excitor field current and independent of the speed. Then the generator voltage varies in a wide range of speed directly proportionally to the speed and the frequency, which relationship is frequently most desirable and permits of dispensing with an amplification of the excitor field current involving the risk of heating. Building up the voltage is already possible at a low speed and particular expedients are not required for this purpose. The high reactance furthermore improves a rapid voltage control in the event of great, abrupt load changes.
Said reactance is preferably at least 2.5 times that of said equivalent resistance.
It should be noted that it is known from Dutch Pat. No. 88,339 to choose a given ratio between the reactance of part of the current circuit and the equivalent resistance of the supply rectifier and the energizing winding with a synchronous generator having a device for self-excitation and automatic control of the energizing current.
In order to further improve the controllability of the generator it is preferred to connect a circuit branch with variable impedance parallel to the rectifier feeding the excitor, to proportion the magnitude of the component determining the fed-back current so that at the nominal speed the energizing current determined by said component would produce a slight voltage excess of the generator and to control the controllable impedance so that at the nominal speed the energizing current is reduced to the value required for producing the desired generator voltage. The excessively strong feedback or, in other terms, the increase in circulating amplification of the control-circuit provides a rapid control of the generator. This property becomes manifest, for example, in the start and in the fast post-control of the generator voltage in the event of load shocks. This applies particularly when the controllable impedance has a slight inertia. An example of such a rapidly controllable impedance is a thyristor, which is controlled in the present device from the generator output.
The commonly rotating rectifier is bridged by a voltage limiter in order to protect it against excessive voltages, for example, in a non-synchronous operation of the generator. When choosing the great reactance of the multi-phase winding according to the invention the voltage limiter in a preferred embodiment will be formed by thyristors. Owing to the high reactance of the multi-phase winding it may be considered as a current source and in the event of a short-circuit on the d.c. side the current passing through the rectifier varies only little. In the event of excessive voltage the thyristors break down and convey current. Owing to the small resistance the dissipation is slight and does not give rise to difficulties. Moreover, the thyristors occupy only little space.
It is possible to control the voltage value at which breakdown occurs by connecting the control-input of a thyristor to a network wound parallel to the field winding. However, this control-input may remain open.